JPH09195005A - Austenitic heat resistant steel excellent in high temperature strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an austenitic heat resistant steel excellent in high temp. strength such as creep rupture strength and having performance suitable for the material for apparatus used in a severe high temp. environment. SOLUTION: This steel contains, by weight, 0.05 to 0.15% C, <=0.3% Si, <=2% Mn, 17 to 25% Cr, 7 to 23% Ni, 2 to 4.5% Cu, 0.1 to 0.8% Nb, 0.001 to 0.01%. B, 0.05 to 0.25% N, 0.003 to 0.03% sol.Al, 0 to 0.015% Mg, 0 to 2% Mo, 0 to 4% W, total 0.01 to 0.25% of one or >= two kinds among Y, La, Ce and Nd, and the balance Fe with inevitable impurities.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic heat-resisting steel having excellent high-temperature strength such as creep rupture strength and having performance suitable as a material for high-temperature equipment.

[0002]

2. Description of the Related Art Conventionally, SUS304 has been used as a material for equipment such as boilers and chemical plants used under high temperature environment.
18Cr-8N such as H, 316H, 321H, 347H
i-based austenitic stainless steels have been used. However, in recent years, such high temperature environmental conditions have tended to become higher in temperature and pressure, and the operating conditions of the apparatus have become extremely severe. Along with this, higher performance has been required for the materials used. Therefore, 18Cr-8Ni which has been used conventionally is used.
System austenitic stainless steels are no longer able to meet the requirements in terms of high temperature strength.

In general, as a method for improving the high temperature strength of austenitic stainless steel, a means for strengthening precipitation by depositing carbonitride in a matrix is adopted. Further, in addition to precipitation strengthening, solid solution strengthening by adding a large amount of Mo or W and solidifying these elements is also an effective improvement method. The former countermeasure against precipitation strengthening alone has insufficient strength as a material used in a severe high temperature environment such as a superheater tube of an ultra-supercritical pressure power generation boiler. In addition, in the latter measure using both precipitation strengthening and solid solution strengthening, it is inevitable to increase the content ratio of expensive Ni in order to stabilize the austenite structure. Therefore, an increase in manufacturing cost is unavoidable, which has been a major obstacle in terms of economy.

Therefore, the inventors of the present invention conducted a study focusing on N (nitrogen) -added steel as a means capable of suppressing the cost increase and increasing the high temperature strength. As a result, a material having excellent high-temperature strength and microstructure stability was developed.
It was proposed as Japanese Patent Laid-Open No. 2-133048. This material is
It is characterized in that Cu and B were added to the austenitic stainless steel containing N and the contents of Si and Al were suppressed low. However, in recent years, the use environment has become more severe as described above, and in order to reduce the equipment cost, it is required to reduce the wall thickness. Therefore, even the austenitic heat-resistant steel proposed in Japanese Patent Laid-Open No. 62-133048 cannot meet the recent strict requirements regarding high temperature strength.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and suppresses the expensive Ni content as much as possible to ensure the stability of the metal structure, thereby making it possible to achieve the creep. Has excellent high-temperature strength such as breaking strength, and has performance suitable for equipment materials used in harsh high-temperature environments,
Moreover, it aims at providing an inexpensive austenitic heat resistant steel.

[0006]

SUMMARY OF THE INVENTION The present inventors have found that expensive N
Research and development was conducted on measures to dramatically improve the creep rupture strength of N-containing austenitic heat-resistant steels on the premise that the i content rate can be suppressed as much as possible and the stability of the metal structure can be secured. . As a result, the following measures significantly improve creep rupture strength, and
It has been found that it is possible to obtain a stable austenitic heat-resistant steel with almost no decrease in strength even when kept at high temperature for a long time.

Austenitic heat resistant steel containing N, S
Both the i and Al contents are reduced and both Cu and Nb are contained.

[0008] A small amount of Y, La, Ce in the basic steel of
And at least one of Nd.

When the above and are combined, an austenitic heat resistant steel excellent in high temperature strength can be obtained.
For the following reasons. Trace amounts of Y, La, Ce and Nd are N
It promotes the precipitation of fine carbonitrides containing b and stabilizes the precipitate. Therefore, the growth coarsening of the Cu precipitation phase, which has an important effect on the high temperature strength, is suppressed, and the high temperature strength is improved.

The present invention is based on the above findings, and the gist thereof is "C: 0.05 to 0.15% by weight, Si: 0.3% or less, Mn: 2% or less, Cr: 17 to 25%, Ni: 7 to 23%, Cu: 2 to 4.5%, Nb: 0.1 to 0.8%, B: 0.001 to 0.01
%, N: 0.05 to 0.25%, sol. Al: 0.
003 to 0.03%, Mg: 0 to 0.015%, Mo: 0 to 2%, W: 0 to 4%, and a total of one or more of Y, La, Ce and Nd. 0.01-0.25%, balance Fe
And austenitic heat resistant steel consisting of unavoidable impurities and having excellent high temperature strength. "It is in.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The action and content of alloying elements constituting the austenitic heat resistant steel of the present invention (hereinafter simply referred to as the present invention steel) will be described below. In addition, the% display of a chemical composition represents weight%.

C: An element effective for improving the tensile strength and creep rupture strength required for heat-resistant steel by forming a carbide. In order to exert its effect, 0.05% or more is necessary. However, the content rate is 0.
When it exceeds 15%, the amount of undissolved carbides in the solution state increases only when the content exceeds 15%, and the effect of improving the high temperature strength cannot be obtained. Further, it lowers mechanical properties such as toughness. Therefore, the range of C content is 0.05 to 0.15.
%.

Si: an element necessary as a deoxidizing element,
It is an element that also contributes to the improvement of oxidation resistance. However, when it is contained excessively, the weldability or hot workability decreases, and in the steel containing N such as the steel of the present invention, the amount of nitrides precipitated during aging or creep increases,
When used at high temperature, it causes deterioration of toughness and ductility.

Therefore, the Si content is set to 0.3% or less. When importance is attached to toughness and ductility, it is preferably suppressed to 0.25% or less. Further, Si in the steel of the present invention
The lower limit of the content is preferably set to 0.03% or more in order to obtain the deoxidizing effect, because the Al content is suppressed to be low.

Mn: Si is an element which has a deoxidizing action like Si and is effective for improving hot workability. However, 2
%, The heat resistance, particularly the oxidation resistance is deteriorated, so the upper limit of the Mn content is set to 2%. In order to obtain the effect of improving the deoxidizing action and the hot workability, it is desirable that the content be 0.05% or more, and the upper limit is preferably 0.5% from the viewpoint of creep rupture strength.

Cr: An element necessary for improving the corrosion resistance such as oxidation resistance at high temperature, steam oxidation resistance, and high temperature corrosion resistance, and the performance thereof improves as the content rate increases. However, if the content is less than 17%, the above effect cannot be obtained. In addition, the upper limit of the Cr content is set to 25% in order to secure the austenite structure because the Ni content is 23% or less as described later. Ni is 23
This is because, in the case of%, if the Cr content exceeds 25%, ferrite or σ phase may be generated. Therefore, the Cr content is determined to be 17 to 25%.

Ni: An alloying element essential for ensuring a stable austenite structure. The optimum Ni content is
It is determined by the contents of ferrite-forming elements such as Cr, Mo, W, and Nb contained in steel and austenite-forming elements such as C and N. In the steel of the present invention, it is difficult to stabilize the austenite structure if it is less than 7%, while if it exceeds 23%, the manufacturing cost rises too much, so the Ni content is 7 to 2.
3%.

Cu: A fine Cu phase is dispersed and precipitated in the matrix during creep, and has the function of improving creep rupture strength. In order to obtain the effect, it is necessary to contain 2% or more. However, if the content exceeds 4.5%, creep rupture ductility and workability deteriorate. Therefore, the Cu content is set to 2 to 4.5%.

Nb: Nb is an element which precipitates as fine carbonitrides and is effective in improving creep rupture strength by strengthening the dispersion of the precipitates. To achieve this effect,
0.1% or more is necessary. On the other hand, if the content exceeds 0.8%, the amount of undissolved carbonitride in the solution state increases, resulting in deterioration of mechanical properties. Therefore, the upper limit is 0.
8%.

N: N improves the high temperature strength and
It has the function of stabilizing the austenite structure. Therefore, it is used as an element substituting a part of Ni, which is an expensive element. However, if the N content is less than 0.05%, a sufficient effect cannot be obtained.

On the other hand, since N has a larger solid solution limit than C, even if the content ratio is relatively large, N is sufficiently dissolved in the solution state. Therefore, the amount of precipitation of nitrides generated during aging is small, and the deterioration of toughness is relatively small. However, if the N content exceeds 0.25%, the toughness after aging decreases. Therefore, the N content is determined to be 0.05 to 0.25%.

Sol. Al: an element added as a deoxidizer, and it is necessary to contain 0.003% or more. However, if it exceeds 0.03%, when it is kept at a high temperature for a long time, precipitation of intermetallic compounds such as σ phase is promoted, and toughness and creep rupture strength decrease. From such a viewpoint, sol. The content rate of Al is 0.003 to 0.
It was set as 03%. Preferably 0.003 to 0.02%
It is.

B: B has a function of strengthening the crystal grain boundary. Further, it forms a compound with C or N, and its fine carbonitride has a dispersion precipitation strengthening action. This action improves the creep rupture strength. In order to exert this effect, 0.001% or more is required. on the other hand,
If the B content exceeds 0.01%, the weldability is impaired. Therefore, the B content is determined to be 0.001 to 0.01%.

Y, La, Ce, Nd: These elements are
The steel of the present invention has the functions of promoting precipitation of fine carbonitrides and stabilizing the precipitates. The fine carbonitride suppresses the growth and coarsening of the Cu phase, which plays an important role in high temperature strength, and thus the creep rupture strength is dramatically improved. In order to exert its effect, it is necessary to contain at least one kind of these elements, and Y, La, Ce and Nd alone are 0.01 or less.
% Or more is required. However, when it is contained excessively, the hot workability is poor and the effect of improving the creep rupture strength is saturated, so the upper limits of Y, La, Ce and Nd are:
It was set to 0.25%. Therefore, the content range of these elements is 0.01 to 0.25%, and more preferably 0.02 to 0.2%.

These elements may be contained alone or in combination of two or more. In addition,
In the steel of the present invention, all of these elements have the same action and effect. Therefore, when two or more elements are contained at the same time, the total content is required to be 0.01 to 0.25%. .

Mo and W: Mo and W are elements added as necessary, and mainly serve as solid solution strengthening elements and have the function of improving creep rupture strength. In order to obtain the effect, Mo is 0.3% or more and W is 0.5%.
It is desirable to make the above.

On the other hand, when Mo exceeds 2% and W exceeds 4%,
The effect of improving creep rupture strength is saturated, corrosion resistance and workability are deteriorated, and these alloy materials are expensive, which is also disadvantageous in terms of economy. Therefore, when these elements are contained, the Mo content is preferably 0.3 to 2% and the W content is preferably 0.5 to 4%. Preferably, Mo
Is 0.5 to 1.5% and W is 1 to 2%.

Mg: An element used as necessary as a deoxidizing agent for molten steel. In the steel of the present invention, when the Al content is low, it is desirable to use it for sufficiently deoxidizing molten steel. In addition, Mg also has an effect of improving creep rupture strength. To obtain these effects, 0.0
It is necessary to contain 01% or more, and if it exceeds 0.015%, workability and weldability deteriorate. Therefore, when Mg is contained, 0.001 to 0.015% is preferable.

The steel of the present invention having the above chemical composition can be produced by the equipment and processes usually used for commercial production. In order to obtain the steel of the present invention having a predetermined chemical composition, an electric furnace, a converter, a vacuum decarburization furnace (VOD)
Furnace) or an argon-oxygen decarburization furnace (AOD furnace) may be used. In particular, the steel of the present invention includes Y, La, Ce, N
Since an alloying element such as d that is easily oxidized is added, it is also effective to vacuum-process molten steel in advance. In the case of producing a steel pipe, the molten steel whose composition has been adjusted is cast into a billet or a steel ingot by a continuous casting method or an ingot making method. The steel ingot is further processed into billets, and these billets are hot pipe-formed, and if necessary, cold rolling, cold drawing, etc. are performed to finish the product steel pipe. When manufacturing a steel material such as a steel plate or a shaped steel, the manufacturing method usually used may be followed.

[0030]

Examples Table 1 shows the chemical composition of the test materials used in this example. Specimen No. 1 to 21 are examples of the present invention, test material N
o. A to G are comparative examples.

[0031]

[Table 1]

Each test material was subjected to a vacuum high frequency induction furnace,
After manufacturing molten steel of each chemical composition and casting into a 20 kg ingot, each ingot was forged, cold-rolled, and further subjected to solution heat treatment at 1200 ° C. to manufacture. The diameter of the ingot is 100 mm, and the test material after cold rolling has a thickness of 10 mm, a width of 60 mm, and a length of 700 mm.

Creep rupture test pieces (diameter 6 mm, gauge length 30) according to JIS Z 2272 from each test material
mm) was sampled and a creep rupture test was conducted. The creep rupture test was carried out in the temperature range of 650 to 750 ° C. by changing the stress and 700 by the Larson-Miller parameter method.
The creep rupture strength was determined at 1000C for 1000 hours.

Table 1 shows the creep rupture strength of each test material. Sample material No. of the present invention example The creep rupture strengths of 1 to 21 are in the range of 18.0 to 21.7 kgf / mm ² and are good. On the other hand, the sample material No. A
The creep rupture strengths of G to G are as follows. 21.0 of E
Excluding kgf / mm ² , 16.5 to 19.5 kgf /
mm ^2, which is generally low. In addition, the sample material No. of the comparative example. Regarding E, since it contains a large amount of Ce exceeding the upper limit of the range of the present invention, the creep rupture strength is high, but it is about the same as that of the present invention example, and the hot workability is poor. Therefore, there is a defect that cracks or the like occur when the product is processed.

The creep rupture strength at 700 ° C. for 1000 hours was compared in more detail with the examples of the present invention and the comparative examples.

In Table 2, the creep rupture was compared between the examples of the present invention having similar chemical compositions other than Y, La, Ce and Nd and containing Y, La, Ce and Nd and the comparative examples not containing these elements. Showed strength. As is clear from Table 2,
It is understood that the inventive example containing 0.01 to 0.25% of at least one element of Y, La, Ce and Nd has a higher creep rupture strength than the comparative example not containing these elements. Thus, Y, La, Ce and Nd
Was confirmed to play a very important role in the steel of the present invention.

[0037]

[Table 2]

FIG. 1 shows Y, La, Ce and N shown in Table 1.
Among the data on d, taking Ce as an example, the relationship between the content of Ce and the creep rupture strength at 700 ° C. for 1000 hours is shown. From FIG. 1, it can be seen that the creep rupture strength is dramatically improved when the Ce content is 0.01% or more. But,
When it was 0.25% or more, the effect of improving the creep rupture strength did not become higher even if the Ce content was increased. Further, if the Ce content exceeds 0.25%, the hot workability and the like deteriorate. Therefore, in the steel of the present invention, the Ce content is set to 0.01 to 0.25%.

As is clear from Table 1 and Table 2 shown above, Y, La and Nd also have the same effect as Ce, and when each element is used alone, it is used alone or in combination. In this case, it is appropriate that the total content be 0.01 to 0.25%.

Such Y, La, Ce and Nd are
It has the function of promoting the precipitation of fine carbonitrides and stabilizing the precipitates. These fine carbonitrides suppress the growth and coarsening of the Cu phase, which plays an important role in high temperature strength, and thus dramatically improve the creep rupture strength of the steel of the present invention.

[0041]

The austenitic heat resistant steel of the present invention is excellent in high temperature strength and also in creep rupture strength at high temperature and long time. Moreover, Ni, which is an expensive alloying element,
Since the metal content is stabilized while the content ratio of is kept as low as possible, it is also advantageous in terms of economy. Therefore,
INDUSTRIAL APPLICABILITY The steel of the present invention is suitable as a material for a device used in a high temperature environment such as a boiler or a chemical plant, and can be used in a wide range.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the content of Ce and the creep rupture strength at 700 ° C. for 1000 hours.

Claims (1)

[Claims] 1. C: 0.05 to 0.15% by weight,
Si: 0.3% or less, Mn: 2% or less, Cr: 17-2
5%, Ni: 7-23%, Cu: 2-4.5%, Nb:
0.1-0.8%, B: 0.001-0.01%, N
: 0.05 to 0.25%, sol. Al: 0.003
~ 0.03%, Mg: 0 ~ 0.015%, Mo: 0 ~ 2
%, W: 0 to 4%, and 0.01 to 0.25% in total of one or more of Y, La, Ce and Nd, with the balance being Fe and inevitable impurities. Austenitic heat resistant steel with excellent high temperature strength.